I have upgraded to photo, but kept my old iPhoto images. There are plenty of them and now space is an issue so I want to delete some. I have successfully moved them to the external hard drive and have deleted them but now cannot find the trash to empty. The trash for the computer is empty but those images did not show up in that trash so I am assuming that iPhoto still has its own trash somewhere...I just don't know where.
When you migrate iPhoto to Photos, Apple does some magic and uses hard links to allow the new Photos app to have its own library, while not taking up twice the amount of space. If you move your new Photos Library over to a new HD, the old iPhoto library is useless.